The Spanish Civil War was fought between 1936 and 1939, and was caused by the rebellion of conservative army officers stationed in Morocco against the democratically-elected government of Liberals, Socialists, Communists and Basque and Catalan nationalists. The attempted coup was only partially successful and half of the country was left under the control of the elected government (The Republicans) and the other half under the control of the rebellious generals (The Nationalists). In the industrial areas of northern Spain, along with Madrid and Valencia, the Spanish population mobilized and defeated the professional soldiers of the generals, forming Worker's Militia's to defend the Republic, usually based round Anarchist, Socialist or Communist Unions. In Nationalist-controlled areas, Fascist and Carlist groups rallied to the cause of the Nationalists (led by future dictator Francisco Bahamonde Franco). The Fascist and Carlist volunteers, along with the Conservative and Catholic-led army, carried out massacres of leftists and anyone suspected of loyalty to the Republican constitution. In "Red Spain" the Republicans burned churches and tried and executed those who had been disloyal to the constitution. Many priests and landowners were suspected of actively supporting the uprising, often with good reason, adding to the anti-Christian persecution.

This war attracted attention by writers and thinkers worldwide. The British writer George Orwell, who later penned classic anti-communist novels entitled Animal Farm and 1984 and wrote an account of his time in Spain called "Homage to Catalonia", volunteered to fight for the Republican government and against Franco’s Nationalists. He volunteered for the libertarian socialist POUM militia, unlike the majority of international volunteers who were members of the Comintern led International Brigades. The Internal divisions within the Republican camp led to violent confrontations between the Anarchists, Libertarian Socialists and the Communist and Socialist coalition that was attempting to undermine the revolutionary left wing nature of the Republic. In spite of these differences behind the lines, the factions never broke the common front against Franco, Anarchist Militias continued fighting the Nationalists even after their leaders had been arrested. Britain, France and the United States remained officially neutral, despite volunteer combatants from those countries actively engaging in belligerency. Germany and Italy sent troops, and crucially planes, to help Franco. Mexico and the Soviet Union provided the Republicans with arms, but no soldiers. The war in Spain divided public opinion all over the world, the left and center supporting the Republic and the right supporting Franco. The exception to this general rule was amongst Catholic leftists, who supported Franco despite his right-wing politics, as the Church urged priests to condemn the republic from the pulpit.

The Civil-war ended in 1939, the Republican defense broken by infighting, and suffering from a lack of arms due to half-hearted Russian support, with the professional soldiers, especially Arab troops, of Franco playing a decisive role in the defeat of the republic. However, the struggle is remembered as the time when democrats from all over the world joined together to fight against the undemocratic Nationalists, losing the battle but preparing the world for the Second World War.